In modern internal combustion engines, particularly automotive engines, light weight and quiet operation are very desirable qualities. The use of an aluminum alloy to replace cast iron as the traditional material of the engine cylinder block meets the first requirement, but can adversely affect the quietness of the engine. Since the crankshaft converts the reciprocating motion of the engine's pistons to the rotary motion needed for locomotion, it is subject to high frequency cyclic loading that may produce noise which can be heard by the vehicle occupants. A major factor in noise generation is the roundness of the bearing surface that retains the rotating crankshaft. This roundness is largely controlled by the machining operations of the bearing supports which include line boring and microfinishing.
Since the crankshaft bearing is retained in two half circular bearing supports, the ideal situation for machining is a common material in the two bearing support half rounds. In the case of a cast iron cylinder block and a cast iron or sintered powder metal steel bearing cap, this is achieved. When an aluminum alloy cylinder block is used with a cast iron or sintered powder metal steel main bearing cap however, the difference in machining characteristics of the two dissimilar metals causes a nonround bore leading to increased noise. The difference in thermal coefficient of expansion of cast iron or steel powder metal and aluminum alloys (aluminum is almost twice as great) causes further out of roundness as engine temperatures fluctuate, especially when under high engine loads.